Long Term Evolution (LTE) Release 8 (Rel. 8) that has been standardized by 3rd Generation Partnership Project Radio Access Network (3GPP) has adopted single-carrier frequency-division multiple-access (SC-FDMA) as an uplink communication scheme (see, Non-Patent Literatures (hereinafter, abbreviated as “NPL”) 1, 2, and 3). SC-FDMA provides a low Peak-to-Average Power Ratio (PARP) and high power usage efficiency for terminals (User Equipment (UE)).
In the uplink of LTE, both data signals (Physical Uplink Shared Channel (PUSCH)) and control signals (Physical Uplink Control Channel (PUCCH)) are transmitted in units of subframes (see, NPL 1) FIG. 1 illustrates an example of a PUSCH subframe structure in the case of normal cyclic prefix. As illustrated in FIG. 1, one subframe consists of two time slots, and a plurality of SC-FDMA data symbols and pilot symbols (which is called Demodulation Reference Signal (DMRS)) are time-multiplexed in each slot. Upon receipt of a PUSCH, a base station performs channel estimation using DMRSs. The base station then demodulates and decodes the SC-FDMA data symbols using the result of channel estimation. Incidentally, Discrete-Fourier-Transform Spread Orthogonal Frequency Division Multiplexing (DFT-S-OFDM), which is an extended version of SC-FDMA, has become available in LTE-Advanced (LTE-A) Release 10 (Rel. 10). DFT-S-OFDM is a method that expands the scheduling flexibility by splitting the PUSCH formed as illustrated in FIG. 1 into two spectrums and mapping the respective spectrums to different frequencies.
DMRSs to be multiplexed with a PUSCH are generated on the basis of a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence excellent in autocorrelation characteristics and cross-correlation characteristics. In LTE, 30 sequence groups each formed by grouping highly correlated CAZAC sequences having various sequence lengths (bandwidths) are defined (see, e.g., FIG. 2). Each cell is assigned one of the 30 sequence groups according to a cell specific ID (cell ID). As a result, the cells are respectively assigned sequence groups having low correlation between the cells.
A terminal generates a DMRS using a CAZAC sequence corresponding to the allocated bandwidth in the sequence group assigned to the cell serving the terminal and multiplexes the DMRS with a PUSCH. Accordingly, highly correlated DMRSs are transmitted from terminals in the same cell while low correlated DMRSs are transmitted from terminals in different cells. Even if interference between DMRSs transmitted at the same timing occurs, the interference can be reduced by the window function method or equalization because of the low intercell correlation of DMRSs. Meanwhile, the terminals within the same cell can be operated without interference by allocating different frequencies or time to the terminals for orthogonalization. In addition, the same frequency or time can be allocated to different terminals (which is called “Multi-user multi-input multi-output” (MU-MIMO)). In this technique, DMRSs of different terminals can be orthogonalized by configuring a different cyclic shift (CS) for each terminal or multiplying two DMRSs of terminals on a PUSCH by different orthogonal cover codes (OCC).
As described above, the reduction of intercell interference using different sequence groups achieves spatial recycling of radio resources. In addition, application of MI-MIMO enables using radio resources efficiently within a cell. In the manner described above, LTE enables highly efficient uplink transmission.
Furthermore, vertical cell IDs, which enable allocation of any sequence group to any terminal regardless of cell ID of the serving cell, are added in LTE-A Release 11 (Rel. 11).
Incidentally, there has been an explosive increase in mobile traffic due to the widespread of smartphones in recent years. Thus, significant improvement in use efficiency of radio resources is required for providing users with stress-free mobile data communication services. In this respect, small cell enhancement, which involves deployment of a considerable number of small cell base stations each forming a small cell, has been studied in LTE-A Release 12 (Rel. 12) (see, NPL 4). Small cell enhancement is advantageous in that the radio resources allocatable by each cell per terminal can be increased by reducing the coverage to reduce the number of terminals per cell and that the data rate of terminals can be improved accordingly. Meanwhile, it is unrealistic to completely cover all areas by small cells. In addition, another problem is that the frequency of handover increases when a high-mobility terminal is connected to a small cell. For this reason, small cell deployment under the coverages of macro cells, each providing a larger coverage, in an overlaid manner has been considered (see, e.g., FIG. 3; sometimes called heterogeneous network (HetNet)). This small cell deployment enables providing large-volume communication to low-mobility terminals in need of a fast data communication service in small cells while eliminating coverage holes and supporting every terminal in macro cells.